1. Field of the Invention
The present invention generally relates to a semiconductor device having a multi-layer copper line. More specifically, the present invention is directed to a semiconductor device in which upper and lower layers are copper lines and a via contact hole is formed therebetween.
2. Description of the Related Art
As the integration density of semiconductor devices increases,circuit lines become more complex, making it difficult to layout and construct these complex circuits on just a single device layer. Accordingly, multi-layer circuit lines and patterns are used to achieve the desired performance result.
While tungsten may be employed as a conductive material for the circuit line, due to its ability to provide high step coverage necessitated by the increasing aspect ratios of present devices, the material of choice for the circuit lines is still aluminum. Despite its many advantages, the aluminum cannot completely fill a contact hole having a high aspect ratio.
Also, generally in high-integration semiconductor devices, the metal line width is reduced while the circuit line distance remains unchanged. As such, short circuits may result from electro-migration (EM) and circuit line resistance may increase. In effect, high-integration semiconductor devices suffer from circuit line resistance and contact resistance problems that do not occur in low-integration semiconductor devices.
To overcome the foregoing problems, copper has been employed as a circuit line material for semiconductor devices. Compared to aluminum, copper has a lower resistance, which mitigates the circuit line resistance and thereby increases the reliability of semiconductor products. Unfortunately, copper is not ideally suited for conventional semiconductor device patterning processes because the etchability of copper is lower than that of aluminum. In addition, copper is easily diffused into silicon and silicon oxide layers, resulting in short circuits or increases in leakage current and parasitic capacitance.
Various approaches have been developed for solving the problems that are caused by the use of copper as a circuit line conductor. One process is known as a damascene process. In the damascene process, a lower layer is etched to form a recess therein. Copper is deposited and planarized using a chemical mechanical polishing (CMP) technique such that the copper only remains in the recess. Thus, it is not necessary to directly pattern the copper. In order to solve the diffusion problem, a method of depositing a thin diffusion barrier layer and then forming the copper can be used.
Referring to FIG. 1, semiconductor devices (not shown for simplicity) are formed on a substrate 10 through a plurality of conventional fabrication processes. A first interlayer insulating layer 11 is formed to cover these devices, which devices will later connect to a circuit line. Accordingly, the first interlayer insulating layer 11 is patterned to form a contact hole for connecting the devices and the circuit line. A thin barrier layer 13 is then formed on the first interlayer insulating layer 11 containing the contact hole. A lower metal line 15 and a contact 17 are formed on the barrier layer 13. A thin barrier layer 19 is formed over the lower metal line 15. A second interlayer insulating layer 21 is then formed on the barrier layer 19. The metal line 15, the barrier layer 19, and the second interlayer insulating layer 21 are made of copper, silicon nitride, and silicon oxide, respectively.
Referring to FIG. 2, the upper surface of the second interlayer insulating layer 21 is patterned to form a recess for an upper metal line 25. More specifically, using a photoresist, openings for the recess are partially patterned to form a via hole. In this case, the barrier layer 19 is also etched to expose a part of the lower metal line 15. A barrier layer 23 is deposited to cover the recess and the sidewall and bottom of the via hole. Copper is then deposited to fill the recess and the via hole. Using a CMP process, the remaining barrier layer and the copper layer are removed to form a via contact and an upper metal line.
In such a multi-layer circuit line using copper, the barrier layer 23 is interposed between the lower metal line 15 and the upper metal line 25 including the copper layer of the via contact. With reference to the current flow, the barrier layer 23 is serially connected between the copper layers (lower metal line 15 and upper metal line 25). The barrier layer 23 is typically made of tantalum or tantalum nitride. Since the conductivity of tantalum or tantalum nitride is lower than that of copper, the resistance at the via contact interface becomes high.
Also, in the copper line, the density and current resistance become high, creating heat which causes electro-migration (EM). As the contact area is reduced in high integration devices, the current density in a via or a contact becomes higher than that in each layer line. The resulting EM can result in a open circuit. In some cases, the EM is intensified by a so-called current crowding phenomenon, in which the current density is crowded (or accumulates) at a corner where the layer line and the via contact intersect at a right angle.
Another problem is that in addition to the barrier layer preventing uniform connection of each layer copper line to a via contact, voids can be created during the formation of the layer.
Methods for solving the above-mentioned barrier layer problems are disclosed in Korea Patent Publication No. 1999-029770 and Japan Patent publication No. 10-261715. According to these methods, a lower copper line, a via hole, and a barrier layer are sequentially formed. The resulting structure is then etched back to remove the barrier layer on a bottom of the via hole. However, since1 the etch-back technique removes the horizontally formed barrier layer on the bottom of an upper copper line in a dual damascene process, another barrier layer must previously have been deposited for preventing diffusion of the upper copper line.